1. Field of the Invention
This invention relates to an improved signal processing system for determining the relative time delay and frequency shift of a signal from an emitter of interest received at two spaced apart locations, and more particularly to a digital signal processing system to identify components of the signal of interest quickly and accurately in the presence of noise and partially overlapping interfering signals.
Signals received by two or more spaced apart receivers (e.g. antennas for electromagnetic signals and microphones for acoustic signals) are commonly used in the art in order to determine the location of the signal emitter. Accurately determining the relative time delay and frequency shift between two received signals is a key step in obtaining accurate emitter position estimates.
Cross-correlation of the signal received at two locations is a well known and commonly used signal processing technique for measuring the relative time delay and frequency shift of a signal. The relative time delay is referred to herein as the time difference of arrival (TDOA) and the frequency shift is referred to as the frequency difference of arrival (FDOA), also known in the art as Differential Doppler. The presence of one or more interfering signals received along with the signal of interest may distort the cross-correlation result degrading the accuracy of the TDOA and FDOA measurements for the signal of interest.
There are several different ways in which another signal can interfere with the signal of interest. These include, signals nominally in adjacent channels in the spectrum with parts overlapping the signal of interest or narrow bandwidth interference superimposed on the signal of interest. The interfering signal can also completely overlay the signal of interest, but this complete overlap case is not the subject of this invention.
2. Description of the Prior Art
In general, the prior art recognizes that digital signal processing systems can realize improved performance in terms of processing time and in terms of accuracy in TDOA and FDOA measurements by eliminating any interfering signals prior to the correlation process step. But, in the prior art, specific information about the signal of interest, including its spectral shape and modulation, is typically used to distinguish it from interfering signals. In an environment dense with interfering signals with many different signal modulations, this and other prior art techniques to separate the signal of interest from interfering signals are complex and not reliable.